Commercial nuclear reactor power plants require a reactor compartment which surrounds the nuclear reactor vessel and its primary coolant piping. In the event of a piping failure, primary coolant would be released from the reactor into the reactor compartment atmosphere.
The reactor compartment is designed to withstand the maximum pressure and temperature resulting from such a "loss of coolant" accident ("LOCA"). A low leakage structure is specified so that radioactivity released from the nuclear fuel rods in the reactor during this LOCA event will be retained in the reactor compartment.
In order to mitigate the consequences of a LOCA and for the reactor compartment to continue to reliably maintain its integrity over a long term period, several supporting systems are required. These include systems to remove heat from the reactor compartment; remove fission products from the reactor compartment; and provide a source of cooling water for injection into the reactor vessel. In addition, containment systems which feature a pressure suppression system need to maintain this capability throughout the LOCA event.
Prior art containment systems have required active components such as mechanical pumps to provide most of the above supporting system functions. The resulting design is complex, costly, and requires timely and correct operator action. In addition, these active components in turn need reliable onsite diesel generators to provide AC power throughout the accident period. As a result, a hazardous situation can occur if these diesel generators are not available due to a common cause failure.
Prior art containments have been large integrated structures which are costly and difficult to construct. Before the reactor can be installed a large portion of the integrated containment structure must first be completed. Such a construction sequence serves to lengthen the overall plant construction period.
Also, most prior art containment designs result in a continual leakage of radioactivity to the environment throughout the accident period since the containment pressure is above atmospheric. This has resulted in the need for controversial provisions for evacuating the public.
Commercialization of boiling water reactors in the U.S. began in the mid-1960's with a dispersed type of containment design, termed the Mark I plant. This design features a steel vessel for the reactor compartment which is interconnected with a separate steel suppression chamber by means of large diameter vent headers and bellows assemblies. Separate structural supports were provided for each vessel.
As advancements were sought to reduce plant costs and simplify construction, a Mark II type of containment was introduced in 1968. U.S. Pat. No. 3,713,968, Kennedy et al, shows a Mark II type containment comprising an integrated reinforced concrete structure. Supporting mechanical systems remove heat from the water pool and separate pumping systems inject water into the reacter vessel.
The arrangement of the Mark I design was not modular and, therefore, vessel fabrication delays or difficulties would directly impact the overall construction schedule. With the Mark II reinforced concrete containment, the contractor had more flexibility and was not limited by the availability of skilled vessel welders.
It soon became evident that the reactor compartment configuration for the Mark II design, even though it was larger than the Mark I plant, was too small to accommodate all of the added structures needed to accommodate pipe whip and jet impingement. Thus, a third generation of pressure suppression containment, Mark III, was introduced to the marketplace.
Major features of the Mark III system include horizontal weir type openings instead of downcomer piping and an annular pool which surrounds the drywell wall. The suppression chamber exterior wall can either be a reinforced concrete structure or a steel shell surrounded by a concrete shield wall.
The prior art designs, however, have the pressure suppression portion of the containment integrated with the reactor compartment thereby impacting the facility construction schedule. Furthermore, each of the prior art designs require active support components, such as pumps, to operate during an accident.
It is therefore an object of the present invention to provide a simple, passive containment system that requires no mechanical support systems and can function after a LOCA without operator intervention.
It is also an object of the present invention to provide a passive containment system which has its pressure suppression function segregated from the reactor compartment portion of the containment structure thus enabling a large fraction of the containment to be fabricated offsite in a modular design to expedite facility construction.
Another object of the present invention is to limit design complexities and analytical difficulties in the system by decoupling the dynamic and hydrodynamic forces occuring during the pressure suppression process from the nuclear island where the reactor compartment is located.
It is a further object of this invention to provide a containment system than can remove soluble fission products and heat from the containment environment throughout the LOCA event and passively transfer heat from the system for an indefinite period of time without operator action.
A further object of the present invention is to provide a system to passively inject water into the reactor to keep the reactor vessel in a flooded state.